The use of EGR as an addition to charge air introduced into cylinders of an engine aids in controlling tailpipe emissions, especially NOx and particulates. A typical EGR loop has an inlet that is in communication with the engine exhaust system and an outlet that is in communication with the engine intake system. An EGR valve controls flow of exhaust gas through the loop from the exhaust system to the intake system.
Depending on the pierce point of the EGR loop to the exhaust system, an EGR cooler may be included in the loop to cool the exhaust before it reaches the EGR valve. The EGR cooler size is a function of the maximum temperature drop that is needed. The larger the maximum temperature drop, the larger the cooler size.
Constraints on available space may also affect the geometry of an EGR cooler and the number of coolers that are needed in a loop to provide the maximum temperature drop.
Because the maximum temperature drop that an EGR cooler is designed to provide is needed typically during only some of part of the total engine running time, other parts of the running time don't require the same EGR cooling capacity. However, because EGR cooler geometry doesn't change as engine operating conditions change, exhaust may at times be cooled to lower temperatures than it otherwise would if the EGR cooler were smaller.
It has been observed from actual engine testing that an EGR cooler sized to provide a certain outlet temperature at maximum heat rejection may lose cooling efficiency as accumulated engine running time increases. Loss of needed cooling efficiency can have potentially unfavorable implications for an emission control strategy. Furthermore, different engine operating conditions create varying degrees of unburned hydrocarbons and soot in engine exhaust.
In order to provide the cooling capacity within available space for handling occasional maximum cooling needs, the cooler in the EGR loop of the tested engine was actually two EGR coolers connected in series. Over time however, the running of the engine was found to cause sticky, soot-like material to be deposited on cooler surfaces. For example, the cooler outlet became noticeably caked with such deposits. The deposits can also occur on the EGR valve, potentially impairing its operation.
The accumulation of the deposits was believed due to a combination of factors including varying degrees of unburned hydrocarbons and soot in engine exhaust and the reduced temperature of EGR leaving the cooler that occurred when the engine was operating in ways that needed less than the cooling capacity that the two EGRs provided.
One way of avoiding such reduced temperatures, and hence discouraging the accumulation of undesirable deposits, would be to add a by-pass around one or both coolers for some lower exhaust temperature situations. Such a modification requires additional hardware and controls, including conduits, fitting, and one or more by-pass control valves.